The rate of 222Rn gas transport through earthen materials controls 222Rn releases to the atmosphere and to indoor environments. The key soil-related parameters characterizing 222Rn transport in earthen materials are the 222Rn diffusion coefficient and the soil air permeability. Simple correlations have been developed for predicting the 222Rn diffusion coefficient and the air permeability of soils based on fraction of water saturation, total porosity, and arithmetic mean particle diameter. Correlations are based on 1,073 diffusion coefficient measurements and 137 soil air permeability measurements. The geometric standard deviations between the correlation predictions and the measurements are 1.98 for the diffusion coefficients and 2.31 for the soil air permeabilities.
This handbook has been prepared to facilitate the design of earthen covers to control radon emission from uranium mill tailings. Radon emissions from bare and covered uranium mill tailings can be estimated from equations based on diffusion theory. Basic equations are presented for calculating surface radon fluxes from covered tailings, or alternately, the cover thicknesses required to satisfy a given radon flux criterion. Also described is a computer code, RAECOM, for calculating cover thicknesses and surface fluxes. Methods are also described for measuring diffusion coefficients for radon, or for estimating them from empirical correlations. Since long-term soil moisture content is a critical parameter in determining the value of the diffusion coefficient, methods are given for estimating the long-term moisture contents of soils. The effects of cover defects or advection are also discussed and guidelines are given for determining if they are significant. For most practical cases, advection and cover defect effects on radon flux can be neglected. Several examples are given to demonstrate cover design calculations, and an extensive list of references is included.
A mathematical model was developed for calculating radon diffusion coefficients from water contents and pore size distributions of soil materials. The model accounts for radon diffusion in the air‐filled and the water‐filled pore space, for radon solubility in water, and for Knudsen diffusion in extremely small air‐filled spaces. The model considers soil pores to be composed of all possible serial combinations of the size increments from a measured pore size distribution. Diffusion coefficients for the resulting composite pores are computed and then used to calculate the overall diffusion coefficient of the soil by assuming parallel diffusion through all of the pore combinations. The resulting diffusion coefficients increase with the median pore diameter, and decrease with increasing soil water contents and with increasing widths of the soil pore size distribution. The model diffusion coefficients compare well with measured coefficients and with empirical calculations and are useful in evaluating the required soil cover thickness for uranium mill tailings reclamation.
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